Make better use of multi-attr arrays

[DPeterK]

Multi-attr arrays are a unique feature of TileDB that downstream tools - such as Iris - are not really set up to use or expect. This means Iris by default does not make efficiency savings from the fact that multiple phenomena are stored in the same array, and could be accessed in a single read operation, rather than multiple successive read operations.

For example, consider the case of deriving wind speed and direction from x-wind and y-wind. If these two phenomena were stored together in the same TileDB array, a single read would suffice to get both x-wind and y-wind points out of the array to calculate a single tile's wind speed and direction. However we cannot currently request such simultaneous reads using Iris or tiledb_netcdf. This we should fix.